The Case of the Missing Amphibians

A recent report provides the most compelling evidence yet that frogs and other amphibians are in serious trouble worldwide

06-01-2004
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Jim Morrison

The quiet descended abruptly.

In the longleaf pine forests of Mississippi, the song of the ornate chorus frog vanished. In the Australian rain forest, gastric brooding frogs fell mute, extinct only nine years after being described. In California’s High Sierra, meadows rich with the call of amphibians such as the Yosemite toad and the yellow-legged frog became eerily silent. And in Costa Rica’s Monteverde Cloud Forest Reserve, the silence was deafening. Scientists monitoring stable populations of 49 amphibian species listened and watched as they crashed in just two years, with 20 native species disappearing completely—including the golden toad, found nowhere else on Earth.

David Wake, a University of California–Berkeley biologist who has studied amphibians for 40 years, recalls the first time he returned to Monteverde after the crash: "I had the perspective of the forest becoming silent, of what had been just an incredible experience suddenly turning into nothing."

Since the late 1970s, Wake and other herpetologists worldwide have watched in dismay as populations of their study subjects plunged perilously or disappeared altogether. (See National Wildlife, October/November 1999.) The bad news has come from a variety of habitats across Europe, Asia, Africa, Australia, and North, Central and South America. Because amphibian populations naturally fluctuate—and because most herpetologists focus on a single region—it took time for the scientists to comprehend the global nature of the crisis.

With last fall’s publication of the Global Amphibian Assessment, a joint project of IUCN–The World Conservation Union, Conservation International (CI) and NatureServe, the problem has become impossible to ignore. A three-year effort involving more than 520 scientists from 60 nations, the assessment concludes that 1,856 of 5,743 known amphibian species—or nearly a third—are threatened with extinction. Since 1980, as many as 122 kinds of amphibians may have become extinct, and 34 extinctions are confirmed.

The news is likely to get worse. According to CI biologist and report coauthor Janice Chanson, participants lacked sufficient data to assess the status of nearly 1,300 amphibian species. "Many of these are also likely to be threatened," she says, "because the reason we don’t know much about them is that they have small ranges in remote and vulnerable places."

The scientists found no single culprit to explain the declines. So far, individual population drops and extinctions have been attributed to habitat loss, including deforestation, pollution and wetlands destruction; overharvesting (in Asia); invasive species such as introduced bullfrogs and trout; a recently discovered fungal disease; or, most often, a combination of these problems. Still, about half of amphibian losses remain unexplained. Among them are extinctions in apparently pristine areas—such as Costa Rica’s Monteverde—which some scientists are beginning to link to global warming. In the United States, says Wake, "the most mysterious disappearances are from seemingly undisturbed national parks in California, where 50 to 90 percent of historical breeding sites are now without frogs."

In other parts of the country, habitat destruction is clearly the biggest problem, and the Mississippi gopher frog provides a sadly typical example. This stocky, three-inch animal ranges in color from black to brown to gray and is covered with dark spots and warts. Adults spend most of their lives underground in longleaf pine forests characterized by open canopies and abundant ground cover. Over the past two decades, the frog’s distribution, which once stretched across the lower coastal plain from east of the Mississippi River in Louisiana to the Mobile River Delta in Alabama, has declined to one or two small breeding ponds in Mississippi’s DeSoto National Forest.

Towson University biologist Richard Seigel began studying gopher frogs in the early 1990s. By that time, he says, their populations already were in decline because of habitat loss in Alabama, Louisiana and Mississippi. In 1991, Seigel asked a research assistant to search Louisiana for breeding sites recorded in a state museum’s records 30 years earlier. "Every one had been either drained, dug out to be a permanent farm pond, covered over by a shopping mall, or had a highway built through it," he says. "We couldn’t find one intact."

Gopher frogs and other amphibians are unusually sensitive to habitat loss and degradation. "Amphibians have smaller range sizes than do birds or mammals, so if there is habitat loss in an area, they are more likely to be affected," says Chanson. A majority of the animals also depend on fresh water for breeding. "It’s not possible for a frog to fly away like a bird or get up and leave like most mammals," she adds. "Any pollution of their habitat is likely to cause populations to die out." And because amphibians have permeable skin and unshelled eggs, even slight changes in moisture or temperature can harm them.

Such sensitivity means amphibians are frequently the first species to show adverse reactions to pollution, emerging diseases, climate change and other environmental problems—the proverbial canaries in the coal mine. Wake recalls the first time someone asked him whether amphibian declines were signs of even bigger problems in the future. "I asked the reporter, ‘What did the miners do when the canary died? They got out of the mine. Where are you going?’"

NWF PRIORITYThe Climate ConnectionUniversity of California–Berkeley biologist David Wake is convinced that climate change, in addition to overzealous logging, is a major culprit behind the disappearance of so many amphibians from the cloud forests of Central America. Depending on the location, global warming has raised the region’s cloud line by about 100 to 500 meters. If the clouds have moved up, asks Wake, what does that do to the cloud forests?

At Costa Rica’s Monteverde, for example, the frog Hyla pseudopuma breeds in ephemeral ponds. Lacking cloud cover they once had, those ponds today dry up almost immediately, and without them, the frogs breed far less successfully than in the past. In 1999, researchers reported in Nature that a crash of amphibian populations a decade earlier at Monteverde was related to changes in cloud cover and animal populations set in motion by atmospheric warming.